BF40 (2018) New Motor Rpm Hunt/Sneeze/Surge

I bought a brand new BF40 in the spring of 2018. From the first time the motor ran, it has really been running "rough" not like any other Honda motor I have ever run. I have talked with Honda and the dealership I purchased it from many times over the last 2 seasons, only to have the exact same problem still occurring. I am hoping that posting in this thread will shine some light on the issue to help me push Honda/Dealership to get the issue resolved. Any help would be greatly appreciated!

Here is a description of the problem:Starting engine from cold is fine, idles OK. Put it into gear and has a bit of a sputter at very low RPM. Add throttle to 3400 RPM, runs smooth for roughly 60sec +/-. RPMs jump to roughly 4000RPM, on its own. Will run that way for a couple of seconds, then drop down to 3000-3100 RPM and then runs "rough". By "rough" I mean a "sneezing" or "sputter", and runs like the engine is very "bogged down". Occasionally without any real pattern I have been able to notice, the RPM will climb back up to 3600-3800 or so, and then drop back down again. Taking throttle down to idle will produce a very rough idle while in gear and an OK idle when in neutral. This will occur like this on/off for as long as the motor is running and warm. When it cools off (overnight) it will do the same thing again. I can reproduce this issue at other RPM's, and still occurs, but the "sneezing" is less noticeable and the RPM jumps are less dramatic.

Things that I/dealership/Honda have checked. Fuel, (always good fresh no ethanol fuel with treatment as recommended by Honda, have run 10+ tanks all with same result). Checked/replaced, spark plugs, fuel diaphragm, filters, fuel lines, fuel pump, squeeze ball, fuel tanks. We have tried 3 different propellers.

I have read through countless threads on this forum and have found them helpful, and am hoping that you all can help me get to the bottom of this! Thanks in advance!

This is not right, your dealership must fix this issue, it's their job and if they can't then they must get tech help from Honda. From my experience something always shows up using proper diagnostics, ie DrH and a fuel pressure gauge
This kind of issue is not easy to diagnose from the description you have provided but at the same time should not be hard to fix using the correct diagnostic procedure.Just a calculated guess, I would be looking at TPS function, O2 sensor values and fuel pressure.Either way, don't accept no fix.

I agree that you really need to escalate this with Honda and request (demand really) that a regional service rep get involved.

Your dealership sounds incompetent to me. Trying different props to solve the symptoms you describe sounds ludicrous!

Also, after two years and they've only done the limited things you've listed is also suspect as to their abilities and knowledge in engine repair.

At the very least I would expect them to "load the parts shotgun" by now if they can't pinpoint it any other way.

Have they given you any printouts from Dr.H? You should have a few by now!

Things that come to mind for these symptoms are:

Idle air control or IAC faulty

TPS faulty

Throttle body faulty

MAP sensor faulty

Temperature sensor(s) faulty

Crankshaft sensor or wiring faulty

Are there any trouble codes?

If not, they should have ALREADY tried to generate some codes by unplugging sensors to see if the ECM responds properly.

The ECM not generating codes when it should indicates that it is probably fuggazi.

You obviously need help that your dealer can't seem to provide. Time for Honda headquarters to step UP and step IN

Good luck and keep us in the know.

Well, I know nothing about this engine, but it sounds a lot like a fuel restriction. Just for grins, try running the engine with the fuel cap loosened in order to eliminate lack of venting in the fuel tank.

Thank you all, great to know that I am not crazy. I have reopened my case with Honda already this year, and will be hopefully in communication with a "case manager" this week. I agree that the dealership is in over their head on this one, they have really done nothing except leave my boat outside and let it get filthy. However, Honda is forcing me to work through the dealership I bought the motor from. In the meantime, yeah, I will try running with gas cap loose.

Nope!
You're not crazy!
And....running with the gas cap loose is another GOOD idea....that the dealer should have already tried!

At some point, if Honda can't provide you with what you have already PAID for...meaning a brand new, under warranty, good running outboard.....

...just as they are forcing you to work with their dealer...you should remind them that you have the option of forcing them to work with your state's legal system.

Wherein, you would have the right to not only recoup the money you have spent to buy the outboard but also any and all funds spent pursuing this problem. INCLUDING loss of use of this item AND the value of the time and energy they have cost you while failing to meet their obligations pursuant to the agreement of sale.

In California we have a "lemon law" regarding the sale of autos that don't...and won't...run right. After a reasonable time they either have to:

Fix it properly.
Replace it.
Give the money back.

Lemon law or not, you still have the right to be similarly compensated for the money you've spent.

Just sayin'

Update: Still getting the run around from Honda and the dealer. Trying to give them the benefit of the doubt due current events. In the meantime....

After more testing this summer, the mystery continues, so if any of you problem solvers have more ideas, please let me know! I have been running the engine with the fuel cap completely loose, without any change. However, I have been able to recreate and exacerbate the issue by the following:

As per my original description, after the engine gets itself into a "bogged" state and starts sneezing/sputtering with the RPMs fluctuating around 3100 RPM, if you slowly add throttle (new step not in original description) the motor will not increase RPMs but will get "louder" like the throttle is opening up but the boat is "stuck". Letting it run that way for 20 seconds, sounding horrible, without increasing throttle any more, eventually the boat will "break loose" and jump up to almost 4800 RPM in an instant.

What do you guys think?

Ask your dealership to try an ECM from another motor. I had one with very similar symptoms, 12hrs old, after thoroughly checking everything I tried a new ECM, problem solved. I have since discovered there have been a few ECM problems with this model.

Just going over it in my head, the noise increase IS likely the throttle butterfly opening. Which leaves me wondering if the motor (ECU) may or may not realize the throttle is opening? The TPS (throttle position sensor) would SUPPOSED to be passing this info on, but what if it had a dirty connection or was sticking?

Like I said, the ECU issue is a known problem, if there were connection issues, highly unlikely on new motor, there would be a fault logged. My turning to a new ecu was the fact that all sensor and running data, fuel pressure etc were reading correctly.

And just for the record, any BF40 ECU will work on this engine irrespective of the year, it does not have to be the part number listed for that engine

Thank you @iang6766 and @AlanHicks. Update is that I finally have the Honda regional rep coming out to the lake to test it with me next week. The dealership remains skeptical that I even have an issue, and I have not been put in contact with the Honda rep directly.

Any advice to make the most of this time with Honda?

Hope you are able to duplicate your issue!

Well I for believe you, I have experienced the same problem and fixed it. Let us know how you go.